Re: glibc 2.21 - Machine maintainers, please test your machines.

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Jan 26, 2015 at 5:39 AM, Torvald Riegel <triegel@redhat.com> wrote:
> On Mon, 2015-01-26 at 05:35 -0800, H.J. Lu wrote:
>> On Mon, Jan 26, 2015 at 5:32 AM, Torvald Riegel <triegel@redhat.com> wrote:
>> > On Mon, 2015-01-26 at 05:25 -0800, H.J. Lu wrote:
>> >> On Mon, Jan 26, 2015 at 4:50 AM, Torvald Riegel <triegel@redhat.com> wrote:
>> >> > way?  If so, then using __alignof__(sem_t) in Chris' patch would be good
>> >> > because then to_new_sem would resolve to a noop in x32 with your
>> >> > alignment change applied.
>> >>
>> >> I don't want a noop.  I want compiler not to generate a noop at all.
>> >
>> > Yes, that's what I mean.  Compiler should figure out that no conversion
>> > is necessary and not emit any code for this conversion.
>> >
>>
>> If it is true, we should drop "&& !defined (_LP64)" in:
>>
>> +#if __HAVE_64B_ATOMICS && !defined (_LP64)
>> +# define to_new_sem(s) ((struct new_sem *)(((uintptr_t)(s) + 4) & -8))
>> +#else
>> +# define to_new_sem(s) ((struct new_sem *)(s))
>> +#endif
>>
>
> Maybe I wasn't clear.  I suggest to keep this, but use
> __alignof__(sem_t) there in such a way that if sem_t is 8-byte aligned,
> the compiler will figure our at compile time that to_new_sem(s) is
> essentially just a cast.
>
> This way, we can keep this code for all archs including tilegx, yet do
> not have any overhead for the conversion on archs that have sufficient
> alignment of sem_t (eg, x32 with your patch).
>
> Sounds good?
>

That sounds good to me.  Please include my sysdeps/x86/bits/semaphore.h
change when you make this change.

Thanks.

-- 
H.J.